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Variations

The following miscellaneous forging operations are briefly presented: coining, heading, piercing, hubbing, cogging, fullering and edging, roll forging, and skew rolling.

Coining

Minting of coins, where the slug is shaped in a completely closed cavity, is an example of closed-die forging. To produce the fine details of a coin, high pressures, and sometimes several operations are needed, while lubricants are not used because they can prevent reproduction of fine die surface details.

Heading

Heading is an example of open-die forging. It transforms a rod, usually of circular cross-section, into a shape with a larger cross-section. The heads of bolts, screws, and nails are some examples of heading. The workpiece has a tendency to buckle if the length-to-diameter ration is too high.

Piercing

The workpiece, either confined to a die cavity or unconstrained, is pierced by a punch to produce a cavity or an impression. The piercing force depends on three factors: the cross-sectional area of the punch and its tip geometry, the flow stress of the material, and the friction at the interface.

Hubbing

It is a piercing process where the die cavity produced is used for subsequent forming operations.To generate a cavity by hubbing, a pressure equal to three times the ultimate tensile strength of the material of the workpiece is needed.

Cogging

Also called drawing out, successive steps are carried to reduce the thickness of a bar. Forces needed to reduce the thickness of a long bar are moderate if the contact area is small.

Fullering and Edging

It is an intermediate process to distribute the material in certain regions of the workpiece before it undergoes other forging processes that give it its final shape.

Roll Forging

A bar is passes through a pair of rolls with grooves of various shapes. This process reduces the cross-sectional area of the bar while changing its shape. This process can be the final forming operation. Examples are tapered shafts, tapered leaf springs, table knives, and numerous tools. Also, it can be a preliminary forming operation, followed by other forging processes. Examples are crankshafts and other automotive components.

Skew Forging

It is similar to roll forging but used for making ball bearings. A round wire is fed into the roll gap and spherical blanks are formed continously by the rotating rolls.